Laser along body tracker (SABOT II)

ABSTRACT

A non-imaging laser-based tracking system for tracking the position of a targeted moving object. The tracking system includes two lasers: a reference laser and a slave laser. Each laser is a weapon, and when locked on a target, single laser effectiveness may be doubled without a thermal blooming performance loss associated with a single laser operating at twice the power. The slave laser beam is dithered relative to the reference laser beam in a direction along the longitudinal axis of the target. The system includes an optical receiver for repetitively scanning the irradiance profile reflected by the target. Since the slave laser beam is dithered relative to the reference laser beam, both laser beams will jitter and drift together providing a gain factor of two in average irradiance on the moving target.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of parent patent application,Ser. No. 08/729,108 filed on Oct. 11, 1996 for a LASER ALONG BODYTRACKER (SABOT I), by Peter M. Livingston, now U.S. Pat. No. 5,955,724.

The present application is related to issued U.S. Pat. No. 5,900,620,issued May 4, 1999, entitled: "Magic Mirror Hot Spot Tracker" by PeterM. Livingston, and its contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to systems for tracking a movingobject and, more particularly, to a laser-based system for tracking amoving object that employs two lasers; a reference and slave laser. Eachlaser is a weapon, and when locked together on a target, single lasereffectiveness may be doubled without a thermal blooming performance lossassociated with a single laser operating at twice the power. The slavelaser is dithered relative to the reference laser in a direction along alongitudinal axis of the target. The system includes an optical receiverfor repetitively scanning the irradiance profile reflected by thetarget.

2. Description of the Prior Art

Various types of systems are known for tracking moving objects, such asrockets and missiles. Such systems can be categorized as either imagingor non-imaging systems. Conventional imaging type systems normallyutilize an imaging device, such as an electronic camera, for detectingand tracking the position of a targeted moving object. While suchimaging systems are effective in tracking targeted moving objects, suchimaging systems; are known to have limitations when used in combinationwith high power laser beam weaponry. For example, in such systems, thehigh power laser beam is known to interfere with the imaging systempotentially causing loss of the track of the targeted moving object.Although various systems are known for compensating for such aninterference problem, such systems do not effectively eliminate theinterference.

As such, non-imaging laser type tracking systems have been developed. Anexample of such a system is disclosed in copending U.S. patentapplication Ser. No. 08/631,645, filed on Apr. 2, 1996, now U.S. Pat.No. 5,780,838, entitled LASER CROSS BODY TRACKER (LACROSST), assigned tothe same assignee as the assignee in the present invention. The systemdisclosed in the '645 patent application includes a laser generator forgenerating a single beam of laser energy and a beam steerer for steeringthe beam of laser energy to track a targeted moving object. The beamsteerer steers the beam of laser energy in a oscillatory fashion in twoorthogonal directions at a first dither frequency and a second ditherfrequency, respectively. The system also includes a telescope forreceiving reflected laser energy from the targeted object and detectingthe amount of reflected energy received. The detected energy is filteredto form first and second dither frequencies for each channel. Thefiltered signals are synchronously detected by multiplying each channelby a sinusoidal function derived from the laser mirror generator forthat channel. A bias signal is generated from the received reflectedsynchronously detected power proportional to the beam centroiddisplacement from the target midline which allows the beam steerer tosteer the laser beam to center it on the target, thereby tracking thetargeted object.

Unfortunately, laser based tracking systems are subject to what is knownas thermal blooming. Thermal blooming results in a change in therefractive index of the beam path as a result of heating the beam pathtemperature by the laser. A change of the refractive index creates alens effect that causes the laser radiation to spread relative itsoriginal direction. As such, thermal blooming increases the diameter ofthe laser beam as it moves away from the laser source. A detailedexplanation of the thermal blooming is disclosed in U.S. Pat. No.5,198,607, hereby incorporated by reference.

The problem of thermal blooming also reduces the effectiveness of highpower laser weaponry. In order to overcome the thermal blooming problemfor high power laser weaponry, the '607 patent discloses the use of twoindependent lasers separated by a sufficient distance to preventinterference therebetween, focused onto a single moving object, such asa missile. The '607 patent discloses the use of a known imaging typesystem for tracking the location of the targeted moving object. However,the two laser beams are not under closed loop control in the systemdisclosed in the '607 patent, thereby making it relatively difficult todirect both laser beams to a single spat on the target.

SUMMARY

It is an object of the present invention to solve various problems ofthe prior art.

It is yet another object of the present invention to provide anon-imaging tracking system for tracking the position of a targetedmoving object.

It is yet another object of the present invention to provide anon-imaging type tracking system which reduces the effects of thermalblooming.

Briefly, the present invention relates to a non-imaging laser basedtracking system for tracking the position of a targeted moving object.The tracking system includes two lasers: a reference laser and a slavelaser. Each laser is a weapon, and when locked together on a target,single laser effectiveness may be doubled without a thermal bloomingperformance loss associated with a single laser operating at twice thepower. The slave laser beam is dithered relative to the reference laserbeam in a direction along the longitudinal axis of the target. Thesystem includes an optical receiver for repetitively scanning theirradiance profile reflected by the target. Since the slave laser beamis dithered relative to the reference laser beam, both laser beams willjitter and drift together providing a gain factor of two in averageirradiance on the moving target.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantages of the present invention willbecome readily apparent upon consideration of the following detaileddescription and attached drawing, wherein:

FIG. 1 is a diagram illustrating the reflected radiation from auniformly reflecting body illustrating two overlapping laser spots.

FIG. 2A is a graphical representation of the irradiance profile as afunction of the distance along the reflected body for twonon-overlapping laser spots.

FIG. 2B is a graphical illustration of the irradiance profile as afunction of the distance along the reflecting body where the twooverlapping laser spots are overlapping as illustrated in FIG. 1.

FIG. 3 is a diagram of a single laser beam overlapping a highlyreflecting feature on a target.

FIG. 4 is an overall block diagram of the non-imaging laser trackingsystem in accordance with the present invention.

FIG. 5 is a block diagram of an optical receiver and image derotator inaccordance with the present invention.

FIG. 6 is a block diagram of a signal processor for use with thetracking system in accordance with the present invention.

FIG. 7A is a graphical illustration of the received scan signal for fourcomplete scan cycles of the tracking system in accordance with thepresent invention.

FIG. 7B is a differentiated version of the signal illustrated in FIG.7A.

FIG. 8 is a graphical illustration of the power spectral density of thesynchronously detected scan signal at twice the frame rate for thetracking system in accordance with the present invention.

FIG. 9 is a graphical illustration of the detected dither modulationsignal for the tracking system in accordance with the present invention.

FIG. 10 is a graphical illustration of an error characteristic signalfollowing the second detection of the 5 Hz dither modulation signal.

FIG. 11 is a block diagram of an alternative signal processor for usewith the tracking system in accordance with the present invention.

FIG. 12 is a graphical illustration of the scan mirror position as afunction of detector voltage for the signal processor illustrated inFIG. 11 for a condition when the reference and slave spots are perfectlysuperimposed.

FIG. 13 is similar to FIG. 12 but illustrating the detector voltagesignal of FIG. 12 multiplied by a phase lagged sine signal operating atthe scan mirror frequency.

FIG. 14 is similar to FIG. 12 but for the condition when the slave spotleads the reference spot.

FIG. 15 is similar to FIG. 13 illustrating the detector voltage signalof FIG. 14 multiplied by a phase lagged sine signal operating at thescan mirror frequency.

FIG. 16 is similar to FIG. 12 but for a condition when the slave spotlags the reference spot.

FIG. 17 is similar to FIG. 13 but for the condition when the slave spotlags the reference spot.

FIG. 18 is a graphical illustration of the error signal as a function ofthe reference slave spot displacement.

FIG. 19 is a graphical illustration of the settling characteristics ofthe system illustration in FIG. 11 at a time t=o.

DETAILED DESCRIPTION

The present invention relates to a non-imaging laser based trackingsystem for tracking a targeted moving object. In order to minimize theeffects of thermal blooming, the tracking system in accordance with thepresent invention employs two laser beams to detect the location of atargeted moving object. One laser beam is configured as a reference beamand is directed toward the targeted moving object. The other laser beamis configured as a slave and is dithered (oscillated with a smallamplitude) relative to the reference laser beam on the target surface ina direction generally parallel to a longitudinal axis of the target. Asshown in FIG. 1, such a system will result in overlapping laser spots onthe moving target. Since the reference and slave laser beams are lockedtogether, both beams will jitter and drift together producing a gain inthe average irradiation profile of two as illustrated in FIG. 2B. Moreparticularly the irradiance for separated laser spots on a target isillustrated in FIG. 2A. By causing the laser spots on the target tooverlap as illustrated in FIG. 1, the irradiance gain illustrated inFIG. 2B is about twice the normal gain as illustrated in FIG. 2Aassuming equidistant laser paths and equal laser powers.

FIGS. 1 and 2 illustrate the irradiation for a uniformally reflectingbody. FIG. 3 illustrates the use of the invention with a non-uniformallyreflection body. In particular, referring to FIG. 3 a single laser beamcan be locked on to shiny or dull feature of a target. Even though thetotal number of photons scattered from the target is constant, theinvention will process the information as to lock the single beam ontothe distinguishing feature as will be discussed in more detail below.

A system for directing a reference laser beam at the target of interestis described in U.S. Pat. No. 5,780,838 assigned to the same assignee asthe assignee of the present invention and hereby incorporated byreference. The present invention as described below is related tolocking a second laser beam (slave beam) relative to the reference laserbeam on the target surface in a direction generally parallel to thelongitudinal axis of the target.

Referring to FIG. 4, a targeted moving object such a missile or rocket20 is illustrated defining a longitudinal axis 22. A first laser 24 isused to direct a first laser beam 26 toward the target 20. For purposesof illustration herein, the first laser 24 is designated as thereference laser. In accordance with an important aspect of theinvention, a second laser 28 is used to direct to a second laser beam 30toward the target. The second laser beam 30 is dithered (i.e. oscillatedwith a relatively small amplitude) in a direction as indicated by thearrows 32 that is generally parallel to the longitudinal axis 22 of thetarget.

The tracking system forms a closed loop that forces the laser spots fromthe laser beams 26 and 30 to overlap as illustrated in FIG. 1 for alldisturbance frequencies falling within the loop bandwidth. The closedloop system in accordance with the present invention is generallyidentified with the reference numeral 34 and includes an opticalreceiver 36, a signal processor 38, a dither generator 40 and a dithermirror 42. Unlike the single laser tracking system disclosed in thecopending application Ser. No. 08/631,645 for the LASER CROSS-BODYTRACKER (LACROSST), now U.S. Pat. No. 5,780,838 the tracking system 34does not depend on the time variation of the total number of scatteredphotons but rather information derived competitively scanning theirradiance profile created the two laser beams 26 and 30 directed towardthe target 20. The signal processor 38, as will be discussed below inmore detail is used to process the scan data from the optical receiver36 in order to lock the beam 30 from the slave laser 28 to the beam 26,generated by the reference laser 24.

The optical receiver 36 is illustrated in detail in FIG. 5. The opticalreceiver 36 is used to detect the irradiance profiles for example asillustrated in FIG. 2B, reflected from the target 20 by imaging a target20 with its laser spots at the laser site using a rocking mirror scannerassembly as discussed below. The optical receiver 36 includes a primaryafocal telescope 44 for receiving scattered laser energy from arelatively wide field-of-view, for example, a field of regard of severalhundred microradians. The scattered laser energy from the target 20 isdirected by the afocal telescope 44 to an image derotator 46. The imagederotator 46 is used to cause a rocking mirror scanner assembly 48 toscan the scatter laser energy in a direction generally parallel to thelongitudinal axis 22 of the target 20. More particularly, the opticalreceiver 36 includes a single detector 50 having a field stop slit 52which defines the system instantaneous field-of-view. The rocking mirrorscanner assembly 48 causes the image of the overlapping spots on thetarget 20 to be swept over the slit 52. The image derotator 46 thus isused to force the dither direction to be generally perpendicular to thefield stop slit 32. The field lens 54 is used to direct the image on thesingle detector 50. In order to improve the signal to noise ratio, azoom lens assembly 56 may be used in conjunction with a system radarrepresented by the dashed line 58, to fill the field stop slit 52 to thegreatest possible extent. In order to ensure that the spot image on thedetector 50 remains centered, bias signals from the signal processingsystem 38 may be used. The bias signals may be generated by the signalprocessing system 38 resulting from the action of the tracking systemservoloop 34. The action of the servoloop causes the optical signal tobe centered on the detector and eliminates signal drift perpendicular tothe direction of scan.

The optical receiver 36 may be mounted to the coarse gimbals of a laserbeam pointer system. The laser beam pointer system is described indetail in U.S. patent application Ser. No. 08/631,645, filed on Apr. 2,1996, now U.S. Pat. No. 5,780,838, assigned to the same assignee as thepresent invention and hereby incorporated by reference.

Various signal processing systems are suitable for use with the presentinvention. One such signal processing system is identified with thereference numeral 38 and illustrated in FIG. 6. An alternate signalprocessing system is illustrated in FIG. 11 and discussed below.

Referring to FIG. 6, scan mirror generator 53 (FIG. 6) as well as thescan mirror drive 55 form part of the signal processor 38 discussedbelow which drives the rocking mirror scanning assembly 48. The scanmirror generator 53 causes the scan mirror to move an approximatelyconstant angular rate scanning the image back and forth over the fixedslit 52. The detector 50 thus records a voltage proportional to theirradiance filling the slit 52. The slit width and scan extend togetherwith the zoom lens assembly 56 causes the image to be moved completelyout of the field of view and returned. The detected voltage from thedetector 50 thus represents a running integral of the irradiancedistribution.

The output from the detector 50 is applied to a detector preamplifier52. The signal processor 38 is used to develop a scan signal for eachcomplete scan cycle as illustrated in FIG. 7A. The signal processor 38includes a pair of differential amplifiers 54 and 56 as well as a pairof commutating switches s₁ and s₂. As mentioned above, the scan mirrormoves at a constant angular rate scanning the image back and forth overthe fixed slit 52. In a forward scanning direction, the commutatingswitches s₁ and s₂ cause the signal to be applied to the differentialamplifier 54. In particular, when the commutating switch s₁ is closed, asignal from the detector/preamp 52 is applied to a noninverting input ofa differential amplifier 54 and compared with the output of thedifferential amplifier 56 which is zeroed by a capacitor c₂, connectedbetween the output of the differential amplifier 56 and ground. Adischarge resistor R₂ is used to discharge the capacitor such thatoutput of the differential amplifier 56 is zero when the commutatingswitch s₁ is closed. Thus when the commutating switch s₁ is closed, thesignal from the detector/preamplifier 52 will be positive. Thecommutating switch s₂ is used to connect the output of the differentialamplifier 54 to a differentiator 56 in the forward direction of the scancycle to produce the portion of the signal illustrated in FIG. 7A with apositive slope.

In the return direction of the scan cycle, the commutating switch s₁causes the signal from the detector/preamplifier to be connected to aninverting input of the differential amplifier 57. As mentioned above,the capacitor c₂ in combination with the discharge resistor R₂ causesthe output of this amplifier to be zero prior to the rocking mirrorscanning assembly 48 moving in a return direction a noninverting inputof the differential amplifier 57 is connected to the output of thedifferential amplifier 54. In a return direction, a capacitor c₁ andparallel connected discharge resistor R₁ force the output of thedifferential amplifier to be zero in the return direction. Thus, in areturn direction, the detector preamp signal 52 is merely inverted asillustrated by the negative slope of the scan signal for the last halfcycle as illustrated in FIG. 7.

The scan mirror generator 53 is connected between the commutatingswitches s₁ and s₂ to control their operation. More particularly, thescan mirror generator 53 is coupled to a scan mirror drive which, asdiscussed above causes the rocking mirror scanner assembly 48 to scanthe image back and forth over the fixed slit 52. The scanning mirrorgenerator 53 also controls the operation of the commutating switches s₁and s₂ as discussed above. The waveform illustrated in FIG. 7A, shownfor four complete scan cycles is thus produced at the output of thecommutating switch s₂.

A scan signal, as illustrated in FIG. 7A, is applied to thedifferentiator 59. The differentiator 59 differentiates the scan signalto produce a differentiated scan signal as illustrated in FIG. 7B.Except for the line reversal of the odd half cycles, the differentiatedscan signal illustrated in FIG. 7B is similar to the irradiance profilefor partially overlapping laser spots on a target illustrated in FIG. 2.Since the differentiation eliminates the DC component, operation of thesystem does not depend on accurate DC restoration.

After the scan signal is differentiated, the dither modulation portionof the signal, which shows up an FM component, is recovered. Inparticular the differentiated signal illustrated in FIG. 7B issynchronously detected by multiplying it with a cosine signal from thescan mirror generator 53. More particularly, referring to FIG. 6, theoutput of the scan of mirror generator 53 is applied to a frequencydoubler 58. The output of the frequency doubler 58 is essentially a sinewave at twice the sweep frequency of the scan mirror generator 53. Theoutput of the frequency doubler 58 is applied to a 90° phase shiftingdevice 60 which, generates a cosine signal at twice the sweep rate. Theoutput of the phase shifter device is applied to a multiplier 62.

The power spectral density of the synchronously detected scan signal attwice the frame rate is illustrated in FIG. 8 at a 0.6 beam widthseparation. As shown in FIG. 8, the frame rate is 40 frames or 80 scansper second. Thus, for a scan rate of 80 Hz synchronous detection onlydetects the sidebands 40 Hz above and below the 80 Hz above and belowthe 80 Hz signal within the synchronously detected differentiated scansignal at the dither frequency of 5 Hz.

The synchronously detected differentiated scan signal is applied to alow pass filter 64 as illustrated in FIG. 6. The output of the low passfilter 64 is applied to another multiplier 66 used for synchronousdetection at the dither frequency to recover the signed envelope of thedither signal. A signal from a dither mirror scan generator 68 isapplied to the multiplier 66. The dither mirror scan generator 68 isused to drive a dither mirror drive 70, which, in turn, drives thedither mirror and, in turn, the slave beam 30 at the dither frequency.

FIG. 9 illustrates five synchronously detected 5 modulation signalsshown for 7 values of spot separation 0.0 through 0.6 at 0.1 increments.As shown, the dither modulation envelopes illustrated in FIG. 9 aresigned. Thus, referring to FIG. 1, if the dithered spot is to the leftof the reference spot and moves toward it as the high power beam faststeering mirror 42 (FIG. 4), moves from left to right, the envelope signis positive. However, if the dither spot is to the right of thereference spot, the same motion of the high power fast steering mirror42 causes the sign to be negative.

The sign of the dither modulation signal then may be used as an errorsignal to lock the reference beam 30 from the slave laser 28 relative tothe reference beam 26 from the reference laser 24. As shown in FIG. 10,the error signal changes magnitude with the mean spot displacementmeasured in beam radii. FIG. 10 is an example of the detected 5 Hzdither modulation signal recovered for seven values of mean spotdisplacement. As shown in FIG. 10, the system will cause the beam 30 towalk onto the reference beam 26 for least plus or minus 0.6 beam-radiusseparations. As shown in FIG. 6, this detected signal is applied to anintegrator 70 which forms a closed loop with the dither mirror scangenerator 68 to drive the dither mirror with a signal proportional tothe integral of the detected envelope.

As discussed above, an alternate signal processing system, generallyidentified with the reference numeral 100, is illustrated in FIG. 11. Inthis system, light enters the detector 102 after being gathered by theprimary afocal telescope 44 (FIG. 5) reflected by the scan mirror 48 andpassed through the fixed slit 52. The detector 102 (FIG. 11) convertsthe optical signal to a corresponding electrical intensity signal which,in turn, is amplified by a detector preamplifier 104 and multiplied by asinusoid by way of a multiplier 106 to form a product signal. The outputsignal from the multiplier 106 is applied to an integrator 108 whoseoutput is filtered by a filter 110, for example, a low pass 8-poleButterworth filter, for example as disclosed in "Filtering In The Timeand Frequency Domains", by H. J. Blinchikoff and A. I. Zverev, JohnWiley and Sons, New York, 1975, chapter 3.4, "Maximally Flat Filters",hereby incorporated by reference. The output of the filter 110 isapplied to a driver 112 for driving the scan mirror 48 (FIG. 5) toprovide closed loop control of the scan mirror 48 based on spatialdifference of the reflected radiation of the slave and reference beamspots detected by the detector 102. More particularly, the output of theintegrator 108 forms an error signal which can be used to drive the scanmirror 48. As will be discussed in more detail below, when the slave andreference spots 122 are superimposed, the output of the integrator 108is zero. However, when the reference and slave spots are notsuperimposed, the output of the integrator 108 is a positive or negativeerror signal as illustrated in FIG. 18 which drives the scan mirror 48under closed loop control until reference and slave spots aresuperimposed in which case the error signal is zero.

The signal processing system 100 includes a sinewave generator 114, usedto multiply the detector signal to form a product signal. The sinewavegenerator 114 is also used to drive a sawtooth generator 116. Thesawtooth generator 116 is used to drive the scan mirror 48 as discussedabove. A mirror offset bias 118 allows for adjustment of the center tocenter the selected reference image in the mirror scan.

Multiplying the detector signal by a sinusoid allows for synchronousdetection by dividing the detector signals in the two halves 120 and 122as shown in FIG. 13. The sign of the sinusoid is not important. Eitheran in-phase sinusoid or a sinusoid 180° out of phase relative to themirror may be used. As illustrated herein, the sinusoid selected is 180°out of phase for illustration. Referring to FIG. 12, the detector outputvoltage as a function of scan mirror position is illustrated for acondition when the reference and slave spots are perfectly superimposedover one another. As mentioned above, the detector signal is multipliedby a sinusoid to form a product signal. As shown in FIG. 13, thesinusoid is selected to 180° out of phase with the mirror in order todivide tie product signal into two halves; a first half 120 and a secondhalf 122. During the condition when the reference and slave spots aresuperimposed, the first half 120 of the product signal, assuming thatthe sinusoid is 180° out of phase with the mirror, is negative.Conversely, the second half 122 of the product signal is positive. Asshown in FIG. 13, the product signal is proportional to the firstderivative of the output signal from the detector 102 and, as shown, isformed from the equal positive and negative halves 120 and 122 during anintegration period. As mentioned above, the output of the multiplier 106is then integrated by an integrator 108. For the condition illustratedin FIG. 12, since the areas of the first half 120 and second half 120 ofthe product signal are equal but complementary, the output of theintegrator 108 will be 0 during this condition.

It is to be understood that either spot can serve as a reference or aslave spot. The reference is selected by adjusting the bias to centerthe reference spot so that its peak occurs exactly one half of the totalsweep distance. The other half is designated as the slave. Since eitherlaser can be chosen as a slave, both must be equipped with a suitablefast beam steering assembly but only one need be equipped with theoptical receiver 36 illustrated in FIG. 5.

FIGS. 14 and 16 illustrate alternate conditions when the reference andslave spots are not perfectly aligned. In particular, in FIG. 14illustrates condition when the slave spot leads the reference spot. FIG.16, illustrates the condition when the slave spot lags the referencespot. As shown in FIGS. 15 and 17 multiplying the detector 102 outputsignals by an out of phase sinusoid will result waveforms whose averagevalue is not 0, unlike the condition illustrated in FIG. 12. Thus, inthese conditions, the output of the integrator 108 will provide an errorsignal as a function of the displacement of the reference and slavespots producing an error characteristic, for example, as illustrated inFIG. 18 to cause the slave spot to move in the direction of thereference spot.

FIGS. 12-18 are based on the following data: scan rate: 400 radians persecond; loop gain 70, mirror extent: -4 to +4; capture range limitedto + or -4 displacement units. The actual capture range may somewhatgreater than + or -4. However, the error characteristic turns overapproaching 0 at the extremes. As a result, the settling time of theclosed loop at the extremes becomes very long as the slave spotdisappears out of the scanner mirror field of regard as generallyillustrated in FIG. 19 which illustrates the settling time of the loopat time t=0.

The principles of the invention are applicable to systems having morethan two lasers. In such an application, laser beams with a unique tagare used to distinguish them. For example, a typical mid infrared rangechemical laser is known to have eight strong lines. In such anapplication, eight independent identical chemical lasers with the sevendetectors and processor assemblies as illustrated in FIGS. 5 and 11 areused. Each assembly is coded for one common reference and a slave byplacing a dual bandpass optical filter in front of the detector thatpasses only the laser line wavelength assigned to the reference and theone for the selected slave.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

What is claimed is:
 1. A non-imaging tracking system for tracking amoving object having a longitudinal axis, the tracking systemcomprising:a first laser generating a first laser beam; a second laserfor generating a second laser beam; means for controlling the positionof said first and second laser beams; an optical system for receivingreflected radiation; and means including an integrator for integratingsaid detector signals to generate an error signal for controlling theposition of one or the other of said first and second laser beams underclosed loop control.
 2. The non-imaging tracking system as recited inclaim 1, wherein said integrating means includes means multiplying saiddetector signals by a predetermined waveform to form a product signal,wherein said product for signal is integrated to form said error signal.3. The non-imaging tracking system as recited in claim 2, wherein saidpredetermined waveform is a sinusoidal signal.
 4. The non-imagingtracking system as recited in claim 3, further including a filter forfiltering said product signal.
 5. The non-imaging tracking system asrecited in claim 4, wherein said controlling means includes a scanmirror.
 6. The non-imaging tracking system as recited in claim 5,wherein said controlling means includes means for controlling said ascan mirror as a function of said error signal forming a control loop.7. The non-imaging tracking system as recited in claim 6, wherein saidcontrolling means includes a driver for controlling said scan mirror.